Two-dimensional materials confining single atoms for catalysis

Recent Advances on Two-Dimensional Nanomaterials Supported Single-Atom for Hydrogen Evolution Electrocatalysts

Water electrolysis has been recognized as a promising technology that can convert renewable energy into hydrogen for storage and utilization. The superior activity and low cost of catalysis are key factors in promoting the industrialization of water electrolysis. Single-atom catalysts (SACs) have attracted attention due to their ultra-high atomic utilization, clear structure, and highest hydrogen evolution reaction (HER) performance. In addition, the performance and stability of single-atom (SA) substrates are crucial, and various two-dimensional (2D) nanomaterial supports have become promising foundations for SA due to their unique exposed surfaces, diverse elemental compositions, and flexible electronic structures, to drive single atoms to reach performance limits. The SA supported by 2D nanomaterials exhibits various electronic interactions and synergistic effects, all of which need to be comprehensively summarized. This article aims to organize and discuss the progress of 2D nanomaterial single-atom supports in enhancing HER, including common and widely used synthesis methods, advanced characterization techniques, different types of 2D supports, and the correlation between structural hydrogen evolution performance. Finally, the latest understanding of 2D nanomaterial supports was proposed.

Density functional theory-based screening of Ti4C3O2-loaded single atoms for efficient selective catalytic oxidation of formaldehyde

Indoor formaldehyde (HCHO) pollution poses a major risk to human health. Low-temperature catalytic oxidation is an effective method for HCHO removal. The high activity and selectivity of single atomic catalysts provide a possibility for the development of efficient non-precious metal catalysts. In this study, the most stable single-atom catalyst Ti-Ti4C3O2 was screened by density functional theory among many single atomic catalysts with two-dimensional (2D) monolayer Ti4C3O2 as the support. The computational results show that Ti-Ti4C3O2 is highly selective to HCHO and O2 in complex environments. The HCHO oxidation reaction pathways are proposed based on the Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms. According to the reaction energy and energy span models, the E-R mechanism has a lower maximum energy barrier and higher catalytic efficiency than the L-H mechanism. In addition, the stability of the Ti-Ti4C3O2 structure and active center was verified by diffusion energy barrier and ab initio molecular dynamics simulations. The above results indicate that Ti-Ti4C3O2 is a promising non-precious metal catalyst. The present study provides detailed theoretical insights into the catalytic oxidation of HCHO by Ti-Ti4C3O2, as well as an idea for the development of efficient non-precious metal catalysts based on 2D materials.

HER catalytic activity and regulation of a transition metal atom-anchored BC3 monolayer: a first-principles study

An atomic-level understanding of the hydrogen evolution reaction (HER) on a transition metal (TM) atom-anchored 2D monolayer is vital to explore highly efficient catalysts for hydrogen production. Here, the catalytic activities and modulation of TM atom (Ti, Fe, Cu, Zn, Mo, Ag, Au)-doped BC3 monolayers are investigated by first-principles calculations. Au@BC3 and Fe@BC3 are proven to be potentially excellent HER catalysts. Partial oxidation engineering on Zn@BC3 could improve its performance. Au@BC3 and Ti, Cu and Mo-anchored BC3 with the support of a NbB2 (0001) surface are expected to replace Pt due to the Gibbs free energy changes extremely close to zero. It is revealed that the catalytic activity of the adsorption site is highly related to the degree of charge transfer between the adsorption site and substrate.

Research progress of MOFs/carbon nanocomposites on promoting ORR in microbial fuel cell cathodes

With the rapid development of the economy, energy demand is more urgent. Microbial fuel cells (MFCs) have the advantages of non-toxic, safety, and environmental protection, and are considered the ideal choice for the next generation of energy storage equipment. However, the slow kinetics of oxygen reduction reaction (ORR) on MFC air cathodes and the high cost of traditional platinum (Pt) catalysts hinder their practical application, so there is a need to develop efficient, low-cost, and stable electrocatalysts as alternatives. Recently, metal-organic framework (MOFs) has attracted wide attention in electrocatalysis. Electrocatalysts prepared by the nanocomposite of MOFs and carbon nanomaterials have multiple advantages, such as adjustable chemical properties, high specific surface area, and good electrical conductivity, which have been proven to be a promising electrocatalytic material. In this paper, the latest research progress of metal-organic frames (MOFs) and carbon nanocomposites is reviewed, and the preparation methods and modification of MOFs and carbon nanofibers, carbon nanotubes, and graphene composites are introduced, respectively, as well as their applications in MFC cathode. Finally, the main prospects of MOFs/carbon nanocomposite catalysts are put forward.

Recent Advances in Graphitic Carbon Nitride Based Electro-Catalysts for CO2 Reduction Reactions

The electrocatalytic carbon dioxide reduction reaction is an effective means of combating the greenhouse effect caused by massive carbon dioxide emissions. Carbon nitride in the graphitic phase (g-C₃N₄) has excellent chemical stability and unique structural properties that allow it to be widely used in energy and materials fields. However, due to its relatively low electrical conductivity, to date, little effort has been made to summarize the application of g-C₃N₄ in the electrocatalytic reduction of CO₂. This review focuses on the synthesis and functionalization of g-C₃N₄ and the recent advances of its application as a catalyst and a catalyst support in the electrocatalytic reduction of CO₂. The modification of g-C₃N₄-based catalysts for enhanced CO₂ reduction is critically reviewed. In addition, opportunities for future research on g-C₃N₄-based catalysts for electrocatalytic CO₂ reduction are discussed.

Assembly of copper-clusters into a framework: enhancing the structural stability and photocatalytic HER performance

We synthesized a novel Cu₈ cluster and employed it as a building block to further obtain a copper cluster-based MOF material (CCMOF). Compared with the Cu₈ cluster precursor, the periodically constructed CCMOF exhibited better structural stability, and showed enhanced photocatalytic H₂ evolution performance due to efficient mass/charge transportation benefitting from the ordered porous framework structure. This two-step grafting construction strategy is of great significance to the exploration of copper clusters in catalytic applications.

Through the Self-Optimization process to achieve high OER activity of SAC catalysts within the framework of TMO3@G and TMO4@G: A High-Throughput theoretical study

High-throughput DFT calculations are performed to explore the oxygen evolution reaction (OER) catalytic activity of a series of 2D graphene-based systems with TMO₃ or TMO₄ functional units. By screening the 3d/4d/5d transition metal (TM) atoms, a total of twelve TMO₃@G or TMO₄@G systems had extremely low overpotential of 0.33 ∼ 0.59 V, in which the V/Nb/Ta atom in VB group and Ru/Co/Rh/Ir atom in VIII group served as the active sites. The mechanism analysis reveals that the filling of outer electrons of TM atom can play an important role in determining the overpotential value by affecting the ΔG_O* value as an effective descriptor. Especially, in addition to the general situation of OER on the clean surface of the systems containing the Rh/Ir metal centers, the self-optimization process of TM-sites was carried out, and it made most of these single-atom catalysts (SAC) systems to have high OER catalytic activity. All these fascinating findings can contribute to an in-depth understanding of the OER catalytic activity and mechanism of the excellent graphene-based SAC systems. This work will facilitate the design and implementation of non-precious and highly efficient OER catalysts in the near future.

Metal Sites in Zeolites: Synthesis, Characterization, and Catalysis

Zeolites with ordered microporous systems, distinct framework topologies, good spatial nanoconfinement effects, and superior (hydro)thermal stability are an ideal scaffold for planting diverse active metal species, including single sites, clusters, and nanoparticles in the framework and framework-associated sites and extra-framework positions, thus affording the metal-in-zeolite catalysts outstanding activity, unique shape selectivity, and enhanced stability and recyclability in the processes of Brønsted acid-, Lewis acid-, and extra-framework metal-catalyzed reactions. Especially, thanks to the advances in zeolite synthesis and characterization techniques in recent years, zeolite-confined extra-framework metal catalysts (denoted as metal@zeolite composites) have experienced rapid development in heterogeneous catalysis, owing to the combination of the merits of both active metal sites and zeolite intrinsic properties. In this review, we will present the recent developments of synthesis strategies for incorporating and tailoring of active metal sites in zeolites and advanced characterization techniques for identification of the location, distribution, and coordination environment of metal species in zeolites. Furthermore, the catalytic applications of metal-in-zeolite catalysts are demonstrated, with an emphasis on the metal@zeolite composites in hydrogenation, dehydrogenation, and oxidation reactions. Finally, we point out the current challenges and future perspectives on precise synthesis, atomic level identification, and practical application of the metal-in-zeolite catalyst system.

Angstrom-confined catalytic water purification within Co-TiOx laminar membrane nanochannels

The freshwater scarcity and inadequate access to clean water globally have rallied tremendous efforts in developing robust technologies for water purification and decontamination, and heterogeneous catalysis is a highly-promising solution. Sub-nanometer-confined reaction is the ultimate frontier of catalytic chemistry, yet it is challenging to form the angstrom channels with distributed atomic catalytic centers within, and to match the internal mass transfer and the reactive species' lifetimes. Here, we resolve these issues by applying the concept of the angstrom-confined catalytic water contaminant degradation to achieve unprecedented reaction rates within 4.6 Å channels of two-dimensional laminate membrane assembled from monolayer cobalt-doped titanium oxide nanosheets. The demonstrated degradation rate constant of the target pollutant ranitidine (1.06 ms-1) is 5-7 orders of magnitude faster compared with the state-of-the-art, achieving the 100% degradation over 100 h continuous operation. This approach is also ~100% effective against diverse water contaminates with a retention time of <30 ms, and the strategy developed can be also extended to other two-dimensional material-assembled membranes. This work paves the way towards the generic angstrom-confined catalysis and unravels the importance of utilizing angstrom-confinement strategy in the design of efficient catalysts for water purification.

Porous Materials Confining Single Atoms for Catalysis

In recent years, single-atom catalysts (SACs) have received extensive attention due to their unique structure and excellent performance. Currently, a variety of porous materials are used as confined single-atom catalysts, such as zeolites, metal-organic frameworks (MOFs), or carbon nitride (CN). The support plays a key role in determining the coordination structure of the catalytic metal center and its catalytic performance. For example, the strong interaction between the metal and the carrier induces the charge transfer between the metal and the carrier, and ultimately affects the catalytic behavior of the single-atom catalyst. Porous materials have unique chemical and physical properties including high specific surface area, adjustable acidity and shape selectivity (such as zeolites), and are rational support materials for confined single atoms, which arouse research interest in this field. This review surveys the latest research progress of confined single-atom catalysts for porous materials, which mainly include zeolites, CN and MOFs. The preparation methods, characterizations, application fields, and the interaction between metal atoms and porous support materials of porous material confined single-atom catalysts are discussed. And we prospect for the application prospects and challenges of porous material confined single-atom catalysts.

Noble-Metal-Free Multicomponent Nanointegration for Sustainable Energy Conversion

Global energy and environmental crises are among the most pressing challenges facing humankind. To overcome these challenges, recent years have seen an upsurge of interest in the development and production of renewable chemical fuels as alternatives to the nonrenewable and high-polluting fossil fuels. Photocatalysis, photoelectrocatalysis, and electrocatalysis provide promising avenues for sustainable energy conversion. Single- and dual-component catalytic systems based on nanomaterials have been intensively studied for decades, but their intrinsic weaknesses hamper their practical applications. Multicomponent nanomaterial-based systems, consisting of three or more components with at least one component in the nanoscale, have recently emerged. The multiple components are integrated together to create synergistic effects and hence overcome the limitation for outperformance. Such higher-efficiency systems based on nanomaterials will potentially bring an additional benefit in balance-of-system costs if they exclude the use of noble metals, considering the expense and sustainability. It is therefore timely to review the research in this field, providing guidance in the development of noble-metal-free multicomponent nanointegration for sustainable energy conversion. In this work, we first recall the fundamentals of catalysis by nanomaterials, multicomponent nanointegration, and reactor configuration for water splitting, CO₂ reduction, and N₂ reduction. We then systematically review and discuss recent advances in multicomponent-based photocatalytic, photoelectrochemical, and electrochemical systems based on nanomaterials. On the basis of these systems, we further laterally evaluate different multicomponent integration strategies and highlight their impacts on catalytic activity, performance stability, and product selectivity. Finally, we provide conclusions and future prospects for multicomponent nanointegration. This work offers comprehensive insights into the development of cost-competitive multicomponent nanomaterial-based systems for sustainable energy-conversion technologies and assists researchers working toward addressing the global challenges in energy and the environment.

Crystalline Carbon Nitride Supported Copper Single Atoms for Photocatalytic CO2 Reduction with Nearly 100% CO Selectivity

Single metal atom photocatalysts have received widespread attention due to the rational use of metal resources and maximum atom utilization efficiency. In particular, N-rich amorphous g-C₃N₄ is always used as a support to anchor single metal atoms. However, the enhancement of photocatalytic activity of g-C₃N₄ by introducing a single atom is limited due to the bulk morphology and the excess defects of amorphous g-C₃N₄. Here, we report crystalline g-C₃N₄ nanorod supported copper single atoms by molten salts and the reflux method. The prepared single Cu atoms/crystalline g-C₃N₄ photocatalyst (Cu-CCN) shows highly selective and efficient photocatalytic reduction of CO₂ under the absence of any cocatalyst or sacrificial agent. The introduction of single Cu atoms can be used as the CO₂ adsorption site, thus increasing the adsorption capacity of Cu-CCN samples to CO₂. Theoretical calculation results show that reducing CO₂ to CH₄ on Cu-CCN samples is an entropy-increasing process, whereas reducing CO₂ to CO is an entropy-decreasing process. As a result, the Cu-CCN samples exhibited enhanced photocatalytic CO₂ reduction with nearly 100% selective photocatalytic CO₂ to CO conversion. The mechanism of photocatalytic CO₂ reduction over Cu-CCN samples was proposed based on in situ Fourier transform infrared spectra, X-ray absorption spectroscopy, and density functional theory calculation. This work provides an in-depth understanding of the design of photocatalysts for enhancing active sites of the reactants.

Metallic single-atoms confined in carbon nanomaterials for the electrocatalysis of oxygen reduction, oxygen evolution, and hydrogen evolution reactions

Recent advances of single-atom-based carbon nanomaterials for the ORR, OER, HER, and bifunctional electrocatalysis are covered in this review article.

Versatile Applications of Metal Single-Atom @ 2D Material Nanoplatforms

Recently, emerging 2D material-supported metal single-atom catalysts (SACs) are receiving enormous attention in heterogeneous catalysis. Due to their well-defined, precisely located metal centers, unique metal-support interaction and identical coordination environment, these catalysts serve as excellent models for understanding the fundamental issues in catalysis as well as exhibiting intriguing practical applications. Understanding the correlations between metal-support combinations and the catalytic performance at the atomic level can be achieved on the SACs@2D materials nanoplatforms. Herein, recent advances of metal SACs on various types of 2D materials are reviewed, especially their exciting applications in the fields of chemicals, energy, and the environment. Based on the summary and perspectives, this work should contribute to the rational design of perfect metal SACs with versatile properties.

Highlights of Major Progress on Single-Atom Catalysis in 2017

Single-atom catalysis has rapidly progressed during the last few years. In 2017, single-atom catalysts (SACs) were fabricated with higher metal loadings and designed into more delicate structures. SACs also found wide applications in C1 chemical conversion, such as selective oxidation of methane and conversion of carbon dioxide. Both experimental characterizations and computational modeling revealed the presence of tunable interactions between single atom species and their surrounding chemical environment, and thus SACs may be more effective and more stable than their nanoparticle counterparts. In this mini-review, we summarize the major achievements of SACs into three main aspects: a) the advanced synthetic methodologies, b) catalytic performance in C1 chemistry, and c) strong metal-support interaction induced unexpected durability. These accomplishments will shed new light on the recognition of single-atom catalysis and encourage more efforts to explore potential applications of SACs.

2D MOF induced accessible and exclusive Co single sites for an efficient O-silylation of alcohols with silanes

Cobalt single sites on ultrathin two-dimensional nitrogen doped carbon (Co SAs/2D N–C) derived from 2D metal–organic frameworks (MOF).

2D Oxide Nanomaterials to Address the Energy Transition and Catalysis

2D oxide nanomaterials constitute a broad range of materials, with a wide array of current and potential applications, particularly in the fields of energy storage and catalysis for sustainable energy production. Despite the many similarities in structure, composition, and synthetic methods and uses, the current literature on layered oxides is diverse and disconnected. A number of reviews can be found in the literature, but they are mostly focused on one of the particular subclasses of 2D oxides. This review attempts to bridge the knowledge gap between individual layered oxide types by summarizing recent developments in all important 2D oxide systems including supported ultrathin oxide films, layered clays and double hydroxides, layered perovskites, and novel 2D-zeolite-based materials. Particular attention is paid to the underlying similarities and differences between the various materials, and the subsequent challenges faced by each research community. The potential of layered oxides toward future applications is critically evaluated, especially in the areas of electrocatalysis and photocatalysis, biomass conversion, and fine chemical synthesis. Attention is also paid to corresponding novel 3D materials that can be obtained via sophisticated engineering of 2D oxides.

Catalysis with Two-Dimensional Materials Confining Single Atoms: Concept, Design, and Applications

Two-dimensional materials and single-atom catalysts are two frontier research fields in catalysis. A new category of catalysts with the integration of both aspects has been rapidly developed in recent years, and significant advantages were established to make it an independent research field. In this Review, we will focus on the concept of two-dimensional materials confining single atoms for catalysis. The new electronic states via the integration lead to their mutual benefits in activity, that is, two-dimensional materials with unique geometric and electronic structures can modulate the catalytic performance of the confined single atoms, and in other cases the confined single atoms can in turn affect the intrinsic activity of two-dimensional materials. Three typical two-dimensional materials are mainly involved here, i.e., graphene, g-C₃N₄, and MoS₂, and the confined single atoms include both metal and nonmetal atoms. First, we systematically introduce and discuss the classic synthesis methods, advanced characterization techniques, and various catalytic applications toward two-dimensional materials confining single-atom catalysts. Finally, the opportunities and challenges in this emerging field are featured on the basis of its current development.

C2N-graphene supported single-atom catalysts for CO2 electrochemical reduction reaction: mechanistic insight and catalyst screening

Single-atom catalysts (SACs) have emerged as an excellent platform for enhancing catalytic performance. Inspired by the recent experimental synthesis of nitrogenated holey 2D graphene (C2N-h2D) (Mahmood et al., Nat. Commun., 2015, 6, 6486-6493), we report density functional theory calculations combined with computational hydrogen electrode model to show that C2N-h2D supported metal single atoms (M@C2N) are promising electrocatalysts for CO2 reduction reaction (CO2 RR). M confined at pyridinic N6 cavity promotes activation of inert O[double bond, length as m-dash]C[double bond, length as m-dash]O bonds and subsequent protonation steps, with *COOH → *CO → CHO predicted to be the primary pathway for producing methanol and methane. It is found that *CO + H+ + e- → *CHO is most likely to be the potential determining step; breaking the scaling relation of *CO and *CHO binding on M@C2N SACs may simply be a rare event that is sensitively controlled by the detailed geometry of the adsorbate. Among twelve metals screened, M@C2N SACs where M = Ti, Mn, Fe, Co, Ni, Ru were identified to be effective in catalyzing CO2 RR with lowered overpotentials (0.58 V-0.80 V).

Versatile design and synthesis of mesoporous sulfonic acid catalysts

Mesoporous sulfonic acid catalysts (MSAC) are widely used in acid-catalyzed reactions, including biomass conversions with plenty of polar solvents and precursors. The catalytic efficiency of MSAC is greatly affected by the microenvironment around the sulfonic acid sites. In this review, the progress on modification of microenvironment of MSAC is reviewed over the past decade. Hydrophobic modification allows MSAC prevent the adhesion of water molecules onto sulfonic acid sites, to abate the risk of reduced acid strength and catalytic efficiency. In comparison, hydrophilic properties can bring positive effect on acid-catalyzed reactions with the aid of hydrophilic interaction between polar functional groups on MSAC and hydrophilic groups of specific substrates. Amphiphilic MSAC with tunable wettability for specific substrates and solvents tend to improve the efficiency in certain reactions with mixed solvents or reactants of different polarity, especially for biphasic systems of immiscible liquids. Furthermore, much attention has been attracted on modification of surface to simulate the microenvironment of homogeneous solvents and enzyme biocatalysts in recent research. New trends of this field are also highlighted.